The present invention generally relates to digital communication systems and particularly relates to the use of QAM communication signals in such systems.
Evolving wireless communication standards increasingly focus on achieving higher data rates while maintaining acceptable communication reliability. Such efforts typically involve the use of higher-order modulation methods that are more complex than the modulation standards used in earlier systems. For example, in contrast to the relatively simple constant-envelope frequency modulation adopted in the original Analog Mobile Phone System (AMPS), the developing Wideband CDMA (WCDMA) standards have adopted 16-ary Quadrature Amplitude Modulation (16QAM) for use in the High Speed Downlink Shared Channels (HS-DSCHs) defined by those standards. Other developing third generation (“3G”) and fourth generation (“4G”) wireless communication systems also have adopted some form of higher order QAM, with some systems using or investigating the use of 64QAM and higher.
Receivers, e.g., wireless communication terminals, etc., receiving such signals must “map” the received symbols into a defined modulation constellation corresponding to the particular order of QAM being used. For example, 16QAM defines sixteen constellation points, each defined by a unique pairing of phase and amplitude, and each representing a unique four-bit value. Thus, source information bits are mapped four-at-a-time into corresponding 16QAM modulation symbols that ultimately are transmitted via an associated carrier frequency signal. The receiver's job in simplified terms thus becomes one of determining what symbols were received by evaluating where the received symbols fall in the defined modulation constellation in terms of their amplitude and phase. A nominal 16QAM constellation comprises four rows of four constellation points each, symmetrically distributed about an x-y (real-imaginary) origin at a desired point spacing.
In one type of 16QAM encoding, the modulation symbols are Gray-coded, wherein the binary representations of the respective modulation symbols differ by one bit from neighbor-to-neighbor. Various approaches exist for demodulating Gray-coded QAM. Commonly, rather than making “hard” decoding decisions, e.g., “1” or “0” per bit decisions, receivers employ some form of “soft” decoding wherein the individual bits conveyed by the received QAM symbols are estimated, or otherwise assigned a “confidence” weighting indicating the quality of each bit decision. In the context of Gray-coded QAM, such bit soft value computations may be performed using region-specific equations, wherein the calculation performed to compute a given bit's soft value depends on the particular region of the modulation constellation the received symbol lies in. Such an approach can lead to computational inefficiencies because of the selection logic overhead associated with selecting the appropriate equation(s) to use for each region.
Past approaches have overcome regional solution inefficiencies by propounding simplified soft value equations that span two or more constellation regions, thus obviating the need for per region soft value equations. However, since such approaches are based on simplifying approximations, they do not yield exact solutions in the sense that the bit soft values obtained from carrying out the simplified equations do not match exactly the results that would be obtained by carrying out the full, region-specific equations.